The present invention relates to a fuel pump control apparatus for an internal combustion engine, and more particularly to apparatus for control of the rotational speed of a fuel pump by controlling the supply of electrical power to the fuel pump in accordance with a pulse signal, the duty cycle of which is controlled on the basis of the operating state of the engine.
In a conventional fuel pump control apparatus for an internal combustion engine, for example, as disclosed in unexamined published Japanese Patent Application (Tokkaisho) No. 57-68530, a central processing unit calculates the rotational speed of a fuel pump optimal to the operating state of the engine and outputs a high-level voltage signal, a low-level voltage signal or a high-impedance signal corresponding to the calculated rotational speed via an input/output interface. The engine state is determined on the basis of output signals from operating state sensors, which include an idle switch, a full-throttle switch, and a starter switch. When the output signal from the input/output unit is the high-level voltage signal, a power transistor is turned on and off in accordance with a pulse signal with a fixed duty cycle from a first oscillator to transmit electrical current at a corresponding duty cycle to drive the fuel pump. When the output signal from the input/output unit is a low-level voltage signal, the power transistor is turned on and off in accordance with a pulse signal with a second fixed duty cycle from a second oscillator to transmit electrical current at a corresponding duty cycle to drive the fuel pump. Finally, when the output signal from the input/output unit is a high-impedance signal, neither of the pulse signals from the first and second oscillators causes the power transistor to be turned on and off and the power transistor operates at a 100% duty cycle, thereby supplying maximum electrical current to the fuel pump.
As is obvious from the above, this conventional fuel pump control apparatus requires two different oscillators which output pulse signals with different duty cycles. This requires a correspondingly complicated circuit network, including the necessity of providing two output circuits, one to each oscillator, thereby degrading system reliability and resulting in high manufacturing cost.